The present invention relates to sub-carriers within signals and, more specifically, to measuring sub-carrier frequencies and sub-carrier frequency offsets.
Within a wireless network, such as an Orthogonal Frequency Division Multiplexed (OFDM) network, there are a number of situations in which undesirable frequency offsets can be introduced to a signal between the signal""s transmission, for example from a Base Transceiver Station (BTS), and signal reception, for example at a Mobile Station (MS). For instance, one effect in which a frequency offset may be introduced, commonly known as the Doppler Effect, results from a MS changing locations while communicating with a BTS, this movement causing a change in the trajectory of signals arriving at the antenna of the MS. Further, undesired frequency offsets between transmitted signal frequencies and the frequencies at which signals are recovered may be introduced by clock variations within the Analog-to-Digital Converters (ADC)of the MS or by slight differences in frequencies between the oscillators of the transmitter within the BTS and the receive within the MS.
The result of these introduced frequency offsets is a constellation rotation during the recovery of the signals which can cause errors in the recovered sub-carriers and hence an increase in the Bit Error Rate (BER) for the system. This problem becomes even more significant in cases in which the sub-carriers form part of larger, more complex constellations.
There are well-known techniques for introducing frequency offsets in cases that the frequency offsets are predetermined fixed values. For instance, frequency offsets are purposely introduced with Automatic Frequency Control (AFC) systems implemented within Frequency Modulation (FM) radio broadcast receivers. Unfortunately, the frequency offsets resulting within a wireless network discussed above, such as those due to the Doppler Effect, are not predetermined fixed values but rather are variable values. There are currently no known techniques for measuring these frequency offsets.
One broad aspect of the invention provides a method of identifying frequency offsets which may for example have been introduced after a signal has been transmitted over a wireless channel. The method involves sampling a received signal having an input bandwidth to generate a respective set of samples for each of a plurality of symbol periods; filtering the sets of samples using DWPT filters to produce a plurality of sub-sampled outputs, the sub-sampled outputs each having a respective fraction of the input bandwidth; for each of the sub-sampled outputs, performing a narrowband FFT on the sub-sampled output produced over a plurality of symbol periods to generate a respective set of frequency domain samples; processing each set of frequency domain samples to determine at least one respective frequency offset.
In some embodiments, the received signal has a plurality N of sub-carriers, and N samples per symbol period are taken.
In some embodiments, the received signal is an OFDM signal having a plurality N of evenly spaced sub-carriers, and the symbol periods are OFDM symbol periods. In this case, sampling a received signal having an input bandwidth to generate a respective set of samples for each of a plurality of symbol periods comprises obtaining N samples per OFDM symbol period as the respective set of samples.
In some embodiments, filtering the sets of samples using DWPT filters to produce a plurality of sub-sampled outputs, the sub-sampled outputs each having a respective fraction of the input bandwidth involves filtering the sets of samples using first through Wth stages of DWPT filters in sequence.
The first through Wth stages of DWPT filters may for example collectively comprise DWPT filter stage w, w=1, . . . ,W wherein the wth filter stage comprises 2w DWPT filters each having a bandwidth 1/(2w)xc3x97input bandwidth, with the bandwidth of the 2wfilters in each stage collectively covering the input bandwidth, each of the 2W filters in the wth stage outputting a N/2W samples of a respective one of said sub-sampled outputs.
In some embodiments, for each of the sub-sampled outputs, performing a narrowband FFT on the sub-sampled output produced over a plurality of symbol periods to generate a respective set of frequency domain samples involves performing a narrowband FFT on N samples of the sub-sampled output collected over 2W symbol periods, wherein the respective set of frequency domain samples comprises N frequency domain samples.
In some embodiments, processing each set of frequency domain samples to determine at least one respective frequency offset involves determining N/2W frequency offsets per set of frequency domain samples.
In some embodiments, determining N/2W frequency offsets per set of frequency domain samples involves for each sub-carrier of the OFDM signal, identifying a maximum frequency domain sample in a respective sub-range of one of the sets of frequency domain samples and associating the maximum frequency domain with a respective frequency offset value.
In some embodiments, the method further involves performing an FFT on each set of samples to produce wideband frequency domain samples for each symbol period; and correcting each wideband frequency sample with one of said frequency offsets.
In some embodiments, for each of the sub-sampled outputs, performing a narrowband FFT on the sub-sampled output produced over a plurality of symbol periods to generate a respective set of frequency domain samples is done every symbol period using the sub-sampled output produced over the most recent plurality of symbol periods.
In some embodiments, for each of the sub-sampled outputs, performing a narrowband PUT on the sub-sampled output produced over a plurality of symbol periods to generate a respective set of frequency domain samples is done once every plurality of symbol periods using completely new sub-sampled output produced over the most recent plurality of symbol periods.
Another broad aspect of the invention provides an apparatus having a receiver for receiving a received signal having an input bandwidth and generating a respective set of samples for each of a plurality of symbol periods; a set of DWPT filters for filtering the sets of samples to produce a plurality of sub-sampled outputs, the sub-sampled outputs each having a respective fraction of the input bandwidth; for each of the sub-sampled outputs, a respective narrowband FFT function for performing an FFT on the sub-sampled output produced over a plurality of symbol periods to generate a respective set of frequency domain samples; and frequency offset logic adapted to process each set of frequency domain samples to determine at least one respective frequency offset.
Advantageously, in some embodiments the DWPT method computes accurate (least squares sense) frequency offset estimates deterministically, in a prescribed number of operations, without iterative convergence problems, and with only linear functions and operations. The DWPT method orthogonalizes received OFDM sub-carriers and remedies shifted phase error. By providing successive frequency offset results, the DWPT yields Doppler rate or frequency shift rate of received sub-carriers. The DWPT method can resolve frequency shift offset to any level of precision by increasing the number of filter/subsampling stages. The DWPT method causes an improvement of signal-to-noise ratio (SNR) in each successive filter/subsampling stage, since the FFT cell bandwidth is halved removing half of the accompanying noise. This rise in SNR is advantageous in identifying the true subcarrier or center frequency within any of the plurality of FFT sub-bands.
Other aspects and advantageous features of the present invention will become apparent to those ordinarily skilled in the art upon review of the following description of specific embodiments of the invention in conjunction with the accompanying figures.